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Today, there are nearly 30 plug-ins on the road in North America, and a few more are built each month—although it’s still very difficult for an individual consumer to purchase a PHEV. Nonetheless, it’s instructive to take a look the wide range of performance in PHEVs. Why is there such a range among companies offering conversion services and kits? It’s the batteries.

Current Prius-based PHEVs store between two and seven times the energy stored in a standard Prius battery. Overall, Prius-based PHEVs average between 65 and 95 MPG, with short periods of highway driving at well over 100 MPG.

The more energy in the battery pack, the further the vehicle can go in all electric-mode, and the longer it can “boost” MPG on the highway. Storing more energy isn’t just a matter of putting in a bigger battery—the type of battery that’s used also matters.

PHEV Battery Types

Modest Energy with Lead-Acid Batteries: On one end of the spectrum are Prius-based PHEVs that store modest amounts of energy on-board: roughly 2-3 kilowatt hours (kWh). Many of the PHEVs in this category have been converted by enthusiasts and use lead-acid batteries.

The main advantage of lead-acid technology is that it’s affordable. It’s also reasonably safe and has proven performance—lead-acid has been used for starter batteries in conventional automobiles for decades. But lead-acid batteries don’t last as long as some other types of batteries. They also store less energy, so larger, heavier packs must be used. Today’s Prius-based PHEVs using lead acid batteries store 2-3 times the energy of a regular Prius battery, allowing them (at low speeds) to drive between 8 and 12 miles in all-electric mode. At higher speeds or in mixed driving, “boosted” gas mileage lasts for about 20 miles.

Lots of Energy with Lithium Ion Batteries: At the other end of the spectrum are Prius-based PHEVs have large amounts of on-board energy (as much as 9kWh). These PHEVs use lithium ion (Li-Ion) batteries— the same battery used in portable electronic devices—because they deliver lots of energy in a smaller, lighter package. PHEVs in this category, such as the EnergyCS Prius, can store as much as seven times the energy of a conventional Prius. This amount of extra energy allows a PHEV to drive in all-electric mode for 25-30 miles, and attain “boosted” mileage for longer distances of 40-50 miles.

The downside is cost. In some of these conversions, the battery pack alone costs more than $10,000. Safety is also a concern for some types of lithium ion batteries and, because lithium ion technology is still maturing, questions remain about how long these batteries will last when put through the rigors of running a PHEV.

Mid-range Energy with Nickel Metal Hydride Batters: In the middle are PHEVs with moderate amounts of on-board energy (around 4-5 kWh). Some of these vehicles, such as prototype developed by battery manufacturer ElectroEnergy and CalCars.org, combine additional nickel-metal hydride batteries (NiMH) with the existing NiMH battery pack in the Prius. Since NiMH batteries are used in the current generation of hybrids, their performance, lifetime, and safety characteristics are well-understood. However, NiMH batteries generally store less energy than Li-Ion batteries. Perhaps for that reason, no after-market conversion companies are now working on offering a NiMH PHEV.

As the current group of PHEV companies move closer to offering a product to the public—and if a major carmaker steps up to the plate—the decision of battery techology and its associated energy storage and cost will continue to be a critical factor.